When automatically selecting a gearbox transmission ratio, the velocity of the vehicle over the ground is to be taken into consideration. For this purpose, sensors for measuring the velocity or the acceleration are attached to one or more wheels of the vehicle. A velocity measured directly or determined by integrating a measured acceleration and, in the event that sensors are attached to several wheels, representing a mean velocity is used as a vehicle reference velocity for selecting the transmission ratio of a gearbox. In the event of slip of at least one wheel, the prior-art methods do not provide a suitable vehicle reference velocity for selecting the transmission ratio of the gearbox any more.
Wheel slip is also taken into consideration with antilock braking control systems. Such systems use various mathematical manipulations for estimating the velocity of a vehicle for comparison with the rotational velocity of a given vehicle wheel for calculating a wheel slip of this wheel, wherein the wheel slip is a parameter. DE-T2 689 08 007 describes a method for estimating a reference velocity or reference acceleration by periodic extrapolation. However, this method does not provide a suitable vehicle reference velocity for selecting the transmission ratio of the gearbox.